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Old 9th February 2012, 04:37 AM   #1
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Default curious about a curious connection

I saw an image, maybe out of an old Radiotron handbook?, of a single triode with both its plate and cathode connected to a the output transformer. It said that it caused complete cancellation of DC in the transformer. It also mentioned it had "severe" drive requirements.

Im curious about this. Anybody familiar with it?
Wasn't sure what it's actually called so I wasn't sure how to search for it.

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Old 9th February 2012, 03:35 PM   #2
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It's called Unity Coupling. Popularised by McIntosh.
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Old 9th February 2012, 06:09 PM   #3
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It still requires two tubes and two winding sets to cancel the DC out in the xfmr even for the McIntosh scheme. The severe drive requirement comes from the cathode section acting as a unity gain cathode follower. The McIntosh scheme gives an approx. overall gain of two, so the grid drive signal is whatever drive the tube internally requires plus half the usual plate swing. The Circlotron is another version.

There are some other schemes, using an external inductor, bypass cap, and one tube, that will cancel the DC in the xfmr, but they are not very attractive usually since the inductor has the same DC issue the xfmr originally had.
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Old 9th February 2012, 10:06 PM   #4
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Here is the image I was looking at.
Not sure where it came from.
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File Type: jpg circuit1.jpg (13.2 KB, 107 views)
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Old 9th February 2012, 10:29 PM   #5
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Quote:
Here is the image I was looking at.
I saw that diagram about 10 years ago along with discussions about whether it would work. It will not. For the DC flux to cancel out the two windings must be exactly the same. Since the plate current is equal to the cathode current for DC and AC the signal current in the transformer will cancel too.

Back then I wasn't too sure, so I built it using a toroid. Yes, everything cancels out except the distortion!

There are schemes to bypass the AC around the cathode winding involving capacitors and inductors, but they are far from perfect.

The simplest thing to do is to use a standard push pull transformer with an output tube tied to one side and a CCS tied to the other side. Set the CCS current equal to the tube current.
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Old 9th February 2012, 10:45 PM   #6
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I think that image or similar was in MJ's earlier book editions, but got deleted later. As George said, it doesn't work, cancels the AC too.

There was a thread here, a number of years ago, about a patent that had 3 or 4 schemes to cancel the DC with a similar setup. As I recall, they either needed an extra inductor with DC in it and a cap, and one or two of the versions just plain didn't work anyway.
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Old 10th February 2012, 03:03 AM   #7
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Too bad. Looked interesting. Aren't the plate and cathode in anti phase with each other? I would think that would work.

Have you tried using a CCS like that? Kind of a current mirror.
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Old 10th February 2012, 03:59 AM   #8
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The plate and cathode are AC anti-phase, but so are the DC currents.

If you put a current mirror on the opposite side of a P-P center tapped OT, you get what has been called anti-triode mode. (essentially just a differential stage driving the P-P OT, but with very high gm on one side) There are a bunch of threads on that. Allows one to get SE (single ended) effects using a P-P OT (and doubled power versus SE mode). A very neat technique actually.

A simpler approach (but not quite SE emulation mode though) would be to use a triode on one side and a pentode on the other side of the P-P OT, both driven by out of phase signals.

You can also use a totem pole (stacked) configuration of two tubes to drive a non center tapped OT without DC in the primary. Inconvenient for the filament power for the top tube though since it is swinging around. And requires doubled up HV power supplies.
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Old 10th February 2012, 03:57 PM   #9
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I always thought having mismatched output devices, like a triode on one side and a pentode on the other, would just cause lots of distortion. I've only seen it in Guitar amp designs.

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Old 10th February 2012, 04:44 PM   #10
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You would have to get the transconductances matched up between them and the individual DC biasing (current) matched up too, otherwise one side would dominate and saturate the xfmr. I haven't tried it myself, just have heard it mentioned. Oh, and it would have to operate in 100% class A mode (both tubes conducting all the time). Certainly most, if not all, guitar amps operate in class B or class aB mode, which would lead to serious distortion this way.
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